1. Field of the Invention
This invention in general relates to the field of injection molding and in particular to adjustable hot sprue bushings with interchangeable tips having multiple edge gates for conveying melted plastic from the injection nozzle to mold cavities.
2. Description of the Prior Art
In plastic injection molding processes, a thermoplastic or thermoset molding material is first controllably heated to plasticity in an injection cylinder. Afterwards, the plasticized compound is forced from the cylinder through a nozzle under the influence of pressure generated within the cylinder. On emerging from the nozzle, the plasticized material is conveyed through a hole in a mold plate, usually stationary, by means of a sprue bushing, or sprue for short. Then, the plasticized material directly enters one or more cavities via gates or enters a runner system prior to passing through a cavity gate. Once in the cavity, the material assumes the shape of the cavity and is cooled to a point where it is sufficiently solidified thereby acquiring the desired part shape. The mold is then opened, and the part is ejected or otherwise removed. The entire process is usually automated, with clamping of the mold parts prior to injection, unclamping for part ejection, and removal after cooling, taking place under the control of a microprocessor or other form of automated controller.
In the industry, molding machines are rated by the number of ounces they will inject per piston or screw stroke and/or by the working area that can be clamped against injection pressure. Therefore, the rating sets a particular machine's capacity which can be utilized to fabricate a variety of different sized parts limited only by the maximum rating. This is accomplished by using, among other things, plates which couple cavities with the injection machine. These plates function to retain and position cavities, contain the plastic feed system, and house the temperature control system. To perform these functions, the plates need to be of suitable thickness for the particular requirements of the part to be molded along with consideration for all supporting systems. Obviously, parts with large working areas, molded at high injection pressures, call for robust mold plates to sustain the relatively larger stresses created during their molding cycle. Plates used to make small parts, on the other hand, can be scaled down in size in accordance with the reduced stresses generated while molding them. Parts between the largest and smallest obviously require intermediate sized plates. Because of the need for plates of different thickness to fully exploit machine capacity, the industry has adopted standardized plate thicknesses. These standard mold plates are also available in a variety of lengths to meet the majority of mold requirements and so offer many advantages to those in the industry. These include quick delivery, cost savings over custom construction and high-quality. Additionally, the use of standardized plate thicknesses reduces design time and allow for the predetermined design and supply of other standard components such as leader pins, return pins, locating rings, and sprue bushings.
In this connection, standard length sprue bushings thus have been made available to accommodate standard mold plate thicknesses. These sprue bushings are available in cold and heated configurations. However, for best process performance regardless of part size, it is known to be beneficial to control the temperature of the plasticized compound more or less constant throughout its travel to the cavity. This reduces process problems associated with material degradation due to thermal variability, improves yield by decreasing scrap losses, and increases machine production time by reducing down time due to freeze offs. Even so, present molding machinery does not always provide for precise temperature control to take advantage of its beneficial effects. Indeed, much of the available machinery is still run employing cold sprue bushings which allow the temperature of the resin to be poorly controlled from the time it leaves the nozzle until it reaches a zone in the mold where temperature control is reacquired with, for example, internal heating channels in the mold. More sophisticated practice, apparently not yet universally accepted, does, however, recognize the advantages of controlling temperature by employing hot sprue bushings to convey material from the nozzle to the cavity gate often times through the fixed mold plate, sometimes referred to as the "A" plate, or base.
A variety of approaches for providing heat in these hot sprue bushings have been used. Among these are the use of resistive heating elements and heat pipes such as those described in U.S. Pat. No. 4,034,952 entitled "HOT PLASTIC INJECTION BUSHING" issued on Jul. 12, 1977. In the latter case, the heat pipes are used to transfer heat from electrically powered heater bands located at the nozzle end of the sprue bushing to regions along the bore near the tip.
The known commercially available hot sprue bushings, however, have a number of disadvantages in that molding shops need to acquire a variety of different sizes to match the range of standard "A" plate dimensions adopted by the industry for use in fabricating parts of different scale, and it is expensive and time consuming to customize them. In addition, their reliability is impaired because of failures associated with tip breakage and heating performance.
Consequently, it is a primary object of this invention to provide a highly reliable hot sprue bushing that can easily be adapted for use with the full range of molding plate thicknesses or easily customized to meet the needs of specific jobs where there is not an exact match between off-the-shelf bushings and standard plate thicknesses.
In addition, to the foregoing object of the invention, it is another object to provide a heated sprue bushing having an interchangeable tip provided with a plurality of edge gates.
As is well-known, a gate is the connection between the terminal end of the flow channel, whether the end of a runner or sprue, and the molded part cavity. As such, it must permit enough material to flow into the cavity to fill it out plus provide any additional material required to overcome normal thermal shrinkage. The location of the gate, its type and size strongly affect the molding process and the physical properties of the part.
Gates are classifiable by their size and location with respect to where they connect with the part cavity, or cavities in cases where the mold is a multicavity type for producing a plurality of parts during each molding cycle. Thus, gates are either large or restricted based on size, and center, ring, or edge based on location. With restricted edge gates, the plasticized material is forced into the cavity through a small orifice where it picks up velocity and, as a result, becomes less viscous, allowing it to successfully fill the mold cavity. When the cavity is cooled, the flow ceases, the plastic in the gate quickly solidifies, and the part may be separated from delivery system. For the next cycle, the solidified plastic in the gate, now "frozen", must again be brought up to the temperature where it will again be in a plasticized state capable of flowing for the next cycle. To assure that this happens, those in the art have provided a variety of more or less complicated heated edge gate systems. Consequently, there is a need for a simplified edge gate system capable of being used in multicavity applications and not needing direct heating or complicated mold designs to achieve this. It is, therefore, an additional object of the present invention to satisfy this need.
It is another object of this invention to provide a hot sprue bushing for uniformly controlling the temperature of plasticized material while it travels from the nozzle to the edges of mold cavity(ies).
Other objects of the invention will in part be obvious and will in part appear hereinafter. A full understanding of the invention will best be had from reading the detailed description to follow in connection with the detailed drawings.